This invention relates to dc-to-dc converters, and more particularly to practical extensions or variations of the new optimum topology switching converter and its coupled inductor version disclosed in U.S. Pat. No. 4,184,179 by Slobodan M. Cuk and Robert D. Middlebrook.
In the new switching dc-to-dc converter disclosed in the aforesaid application, two inductances are employed, one in series with the source in an input circuit, and the other in series with the load in an output circuit. An energy transferring capacitance couples the input and output circuits together with one terminal of the capacitance connected to the input circuit and the other terminal of the capacitance connected to the output circuit. The other terminals of the input and output circuits are connected to a common junction. Suitable switching means are provided for alternately connecting the capacitance in series with the input and output circuits, thereby achieving dc-to-dc conversion. The outstanding performance of this converter is that both input and output currents are non-pulsating, although a small switching ripple does appear on their dc (average) values.
Simplification of this new switching converter topology and further improvement of its performance are obtained by coupling the inductors into a single magnetic circuit with two windings as described in detail in the aforesaid application. The coupling of inductances leads to at least a reduction to half of both current ripples. It was also snown that even zero current ripple in either the input or output circuit may be obtained when the transformer (coupled inductors) is adjusted for the condition n=k, where n is equal to the square root of the ratio of the self inductances L.sub.1 and L.sub.2 of the input and output windings, and k is the coupling coefficient between the two windings. However, in practice, especially when mass production and repeatability are important considerations, neither air-gap nor turns ratio adjustment is convenient for achieving the matching condition n=k for zero ripple. On the other hand, one may be satisfied with a relatively large reduction in current ripple (say 100:1 or 1000:1) compared to its uncoupled value, provided the need for any special adjustment is avoided, i.e., provided a fixed air gap is used and no turns ratio adjustment is required. Several configurations which achieve these goals are presented in this application.